The influence of crystal orientation on the high temperature fatigue crack growth of a Ni-based single crystal superalloy

Abstract

Compact-type (CT) fatigue crack growth (FCG) specimens have been prepared from single crystals of SRR99. The macroscopic crack planes {hkl} and propagation directions 〈UVW〉 were (110)[1̄10], (110)[1̄11], (110)[001], (111)[11̄0] and (001)[010]. FCG tests were conducted at 650 and 850°C in air at 10 and 1 Hz at an R ratio of 0.1. At 650°C, (110) and (111) specimens showed a frequency-dependent FCG rate, not apparent for (001)[010], due to increased cuboidal cross-slip and reduced reversibility at lower frequency. Slip homogenization at 850°C prevented this effect. (001)[010] demonstrated a lower Paris regime FCG rate at 850°C than at 650°C due to crack bifurcation at the higher temperature. Conversely, a higher FCG rate occurred for (110) and (111) specimens at 850°C, as crack tortuosity decreased as it changed from propagation along the γγ′ interface at 650°C to precipitate cutting at 850°C. An orientation-dependent FCG rate and fracture path was found. At 650°C orientations able to accommodate crack tip shear on primary slip planes near-parallel with the crack front provided the best FCG resistance. At 850°C the best FCG response was found for the same orientations that now developed large scale ridges as a result of out-of-plane cuboidal slip.